Circuit boards, circuit cards and other electronic devices are manufactured using photolithographic techniques. In producing such devices, typically, at least one layer of a photosensitive material, typically known as a photoresist, is deposited on a substrate. The photosensitive material may then be patterned by exposing it to radiation of certain wavelengths to alter characteristics of the photosensitive material. Typically, the radiation is from the ultraviolet range of wavelengths, although other wavelengths may be used. Preferably, the radiation causes desired photochemical reactions to occur within the photosensitive material coated on the substrate.
Typically, a mask is positioned between the radiation source and the substrate so as to selectively block portions of the radiation emanating from the radiation source, thereby forming a pattern of exposure in the photosensitive material. Preferably, the photochemical reactions alter the solubility characteristics of the photoresist, thereby allowing the removal of certain portions of the photoresist. Selectively removing certain parts of the photoresist allows for the protection of certain areas of the substrate while exposing other areas.
After exposure to radiation, the photoreactive material is then developed in a developer so as to remove either the exposed or unexposed portions of the photoreactive compound, depending upon its nature. After removing portions of the photoreactive compound, the underlying substrate may be processed. According to one method, the substrate is processed by allowing the diffusion of desired impurities through the openings in the photosensitive material into the substrate. Other processes are also known for forming devices of a substrate.
When creating electronic devices as described above, the radiation source, mask, and substrate must be positioned relative to each other to ensure that the pattern of exposure of the photoreactive compound on the substrate is formed in a desired location. Often, the radiation used to form the pattern in the ultraviolet and/or near ultraviolet range of the electromagnetic spectrum. Such wavelengths are not visible or not readily visible to the human eye. Therefore, aligning the radiation source, mask, and substrate relative to each other is often times difficult or impossible.
Although correctly aligning the substrate, mask, and radiation source is always important, it is even more important in producing large circuit boards, circuit cards, and other large devices. Often, large circuit boards, circuit cards and other devices require two passes of the light source and mask over the substrate to expose the entire area of the substrate. In such cases, alignment is even more critical then in cases where only one pass is required to form a pattern over the entire substrate. In other applications, a substrate may need to be realigned with respect to a radiation source on a mask for other purposes, such as to re-expose the substrate to a different pattern of radiation. In any of these applications, it is very important that the substrate mask and radiation source be correctly positioned so as to result in the formation of the correct pattern in the correct position in the photosensitive material on the substrate.
Alignment of the substrate, mask and radiation source typically has been performed mechanically. Such mechanical methods may be imprecise, thereby resulting in lower accuracy and lower yields. Mechanical aligning techniques may include alignment marks formed on the substrate and/or a substrate support for determining the position of the substrate and/or radiation projected on the photosensitive material on the substrate.
Other methods are at least partially electronic. For instance, cameras or other light detectors may be used to sense the position of the radiation impacting upon photosensitive material on the substrate. Other sensors may also be used to detect the radiation. Electronic sensing techniques also may include alignment marks formed on the substrate and/or a substrate support for determining the position of the radiation projected on the photosensitive material on the substrate. Regardless of what type of sensor or detector is used, if such sensors do not detect ultraviolet or near ultraviolet or wavelengths of radiation, they will be useless.
It was particularly in response to the inability of optical detectors to detect the position of a substrate relative to a projected pattern of ultraviolet and/or near ultraviolet radiation that the present invention was developed.